Finholt, Bond and Schlesinger (J. Am. Chem. Soc. 69, 1199, (1947)) describe the preparation of aluminum hydride (AlH.sub.3) in diethyl ether from AlCl.sub.3 and LiH in accordance with the equation: EQU AlCl.sub.3 +3 LiH.fwdarw.AlH.sub.3 +3 LiCl
This reaction, however, easily continues to LiAlH.sub.4 by the following exothermic reaction: EQU AlH.sub.3 +LiH.fwdarw.LiAlH.sub.4
Advisably, according to Ullmann's Encyclopedia of Industrial Chemistry, volume A 13, pages 213-214, aluminum hydride is prepared from lithium alanate by reaction with aluminum chloride or mineral acids in diethyl ether EQU 3 LiAlH.sub.4 +AlCl.sub.3 .fwdarw.4 AlH.sub.3 +3 LiCl EQU LiAlH.sub.4 +HX.fwdarw.AlH.sub.3 +LiX+H.sub.2
(HX=HCl or H.sub.2 SO.sub.4).
The use of expensive lithium aluminum hydride, which is somewhat dangerous to handle, is a disadvantage of this synthesis.
Early on, therefore, attempts were made to replace the complex hydride LiAlH.sub.4 with other lithium-free hydrides, in order to make the AlH.sub.3 accessible in this way.
For example, the U.S. Pat. No. 3,829,390 discloses the preparation of stable chloride-free and lithium salt-free AlH.sub.3 solutions in lower dialkyl ethers using sodium aluminum hydride according to the following equation: EQU 3 NaAlH.sub.4 +AlCl.sub.3 .fwdarw.4 AlH.sub.3 +3 NaCl
However, by avoiding the use of lithium, this method is not freed from having to use alkali aluminum hydride, the synthesis of which is expensive.
There has been no lack of attempts to use magnesium hydride for the preparation of aluminum hydride or compounds derived therefrom.
Wiberg and Bauer came to the conclusion that the reaction with MgH.sub.2 in diethyl ether, which is similar to the Schlesinger reaction, results in the formation of magnesium aluminum hydride having the formula Mg(AlH.sub.4).sub.2 (Zeitschrift fuer Naturforschung 5b, 397, (1950) and 4b, 131, (1952) as well as the German patent 845,338). Ashby, however, subsequently proved in Inorganic Chemistry, 9, pages 325-326, (1970) that the MgH.sub.2, produced in this reaction hydrogenolytically from Grignard compounds or diorganomagnesium compounds, reacts with aluminum halide to form halogen magnesium alanate (XMgAlH.sub.4).
The Belgian patent 785,348 discloses a method for the preparation of AlH.sub.3 or Mg(AlH.sub.4).sub.2 by reacting MgH.sub.2 with AlCl.sub.3 in the presence of aluminum in diethyl ether: EQU 2 AlCl.sub.3 +3 MgH.sub.2 +Al.fwdarw.2 AlH.sub.3 +3MgCl.sub.2 +Al EQU 2 AlCl.sub.3 +4 MgH.sub.2 +Al.fwdarw.2 Mg(AlH.sub.4).sub.2 +3 MgCl.sub.2 +Al
Experimental proof to confirm the course of the reaction and of the nature of the products was not given, as was already pointed out by Ashby (loc. at.).
The method of the U.S. Pat. No. 4,832,934 once more takes up the reaction of MgH.sub.2 with aluminum halide in cyclic or aliphatic ethers, the highly active MgH.sub.2, used here, being prepared by a method of the European patent 0 003 564. The reaction of this magnesium hydride, which is accessible particularly by using complex catalyst systems, with aluminum halide leads to compounds of the general formula EQU [Mg.sub.2 X.sub.3 (Ether).sub.y ].sup.+ [AlH.sub.4 ].sup.-
in which X is a halogen, ether is a cyclic or aliphatic ether and y is a number from 0 to 6. It is stated explicitly in the US patent that, despite grinding, that is, despite mechanical activation, this product cannot be prepared using conventional, commercial MgH.sub.2. This is also shown in a comparison example. The use of the complex catalyst system, described in the European patent 0 003 564, thus is essential for the preparation of active magnesium hydride by this method. By using dioxane, it is possible to separate 1 mole of MgCl.sub.2 from the aforementioned product. A product having the formula XMgAlH.sub.4 is obtained. AlH.sub.3 cannot be prepared in this way.
The German Offenlegungschrift . . . (patent application P 42 35 334.5) is concerned with the problem of using inexpensive magnesium hydrides, which, according to the teachings of the U.S. Pat. No. 4,832,934, are not usable because of their inactivity and are obtained without the aforementioned complex catalysts at elevated temperatures from the elements, for the preparation of complex halogen magnesium aluminum hydridohalides. Contrary to the teachings of the U.S. Pat. No. 4,832,934, compounds of the general formula EQU [Mg.sub.2 X.sub.3 (Ether).sub.y ].sup.+ [AlH.sub.4-n X.sub.n ].sup.-
wherein X represents halogen, y a number from 0 to 6, n a number from 1 to 3 and ether an aliphatic or cycloaliphatic ether with 2 to 30 carbon atoms (with the exception of t-butyl ether and 1, 4-dioxane), are obtained by grinding magnesium hydride with aluminum halide in aliphatic or cycloaliphatic ethers with 2 to 30 carbon atoms (with the exception of t-butyl ether and 1, 4-dioxane.
For this method, magnesium hydride (MgH.sub.2), prepared by the method of the German Offenlegungsschrift 40 39 278, has proven to be particularly useful. For the method of the German Offenlegungsschrift 40 39 278, MgH.sub.2, which is added to the magnesium to be hydrogenated before the start of the reaction in amounts of at least 1.2% by weight and with a particle size not greater than 400 .mu.m, is used for the hydrogenation of the magnesium. In the following, this magnesium hydride is referred to as "autocatalytically produced magnesium hydride."